Dynamic range (DR) is defined as the ratio of the saturation photon flux (Φmax) to the minimum detectable photon flux (Φmin, in which the signal to noise ratio (SNR)=1). Therefore, the dynamic range of an image sensor is given by:
                              DR          =                                                    Φ                max                                            Φ                min                                      =                                                            V                  max                                                  V                  noise                                            ·                                                η                  ⁡                                      (                                          Φ                      min                                        )                                                                    η                  ⁡                                      (                                          Φ                      max                                        )                                                              ·                                                g                                      c                    max                                                                    g                                      c                    min                                                              ·                                                T                                      int                    max                                                                    T                                      int                    min                                                              ·                                                A                                      v                    max                                                                    A                                      v                    min                                                                                      ⁢                                  ⁢                              V            noise                    =                                                    V                down                2                            +                              V                pix                2                            +                                                η                                      Φ                    max                                                  ·                                  Φ                  min                                ·                                  T                                      int                    max                                                  ·                                  g                                      c                                          max                      ⁢                                                                                                                            2                                ·                                  A                                      v                    max                                    2                                                                                        (        1        )            in which Vmax is the maximum signal strength of the pixel in volts, Vnoise is the signal strength of the noise in volts, η is the quantum efficiency at a given photon flux, gc is the effective converion gain, Tint is the integration time, AV is the signal chain gain, Vdown2=noise power of the signal chain, and νpix2=noise power of the pixel circuits.
It can be seen from equation (1) that there are several different ways of improving the dynamic range of an image sensor. However, each method has its limitations.
A common technique of increasing dynamic range involves changing Tint based on illumination level. However, this method causes severe motion-related imaging artifacts.
Increasing the gain of the signal change AV can be used to increase dynamic range only if the imager system is limited by the resolution of the analog to digital converter (ADC). It does not provide any intrinsic dynamic range increase because it does not increase the number of photons captured, nor reduce the noise levels.
Increasing Vmax or increasing the reset voltage of a photodiode in order to increase the dynamic range of a photodiode in a pixel is another means for improving the dynamic range of an image sensor. A technique of this sort is taught by international application PCT/US2005/026772, published as WO 2006/015113 A1, for which the present inventor is an applicant and inventor and the disclosure of which is incorporated by reference into this specification for all purposes permitted by law and regulation. However desirable a larger Vmax or reset voltage may be, state-of-the-art CMOS fabrication processes do not allow Vmax to be increased due to a steady reduction in the maximum usable power supply voltage. Accordingly, this technique has limitations on when it may be used.
Another method of increasing the dynamic range is to reduce the conversion gain. However, in such cases, low-light-level sensitivity is compromised. The output voltage swing of the image sensor is usually fixed and relatively small due to the power supply scaling. Obtaining a high dynamic range therefore generally requires a small conversion gain so that the input pixel electron accumulation capacity is increased. On the other hand, a small conversion gain increases the reset noise, with reset noise Ne (in electrons) being given by:
                              N          e                =                                            ϕ              T                                      g              c                                                          (        2        )            where gc is the effective conversion gain, and ΦT(=kT/q) is the thermal voltage. Thus, an image sensor (imager) designed with a small conversion gain provides a large signal handling capacity but poor noise, while that with a large conversion gain provides better low-light imaging capability but with a reduced dynamic range.
For a fixed Vmax, the best and the most difficult way of improving the dynamic range without introducing imaging artifacts or increasing the imager noise floor is to change the conversion gain (gc) based on the illumination level. Moreover, the variation in conversion gain must not affect the linearity or calibration capability of the image sensor.
Variation of the conversion gain based on illumination level can solve the problem, but introduces a large number of other problems such as loss of photometric calibration (because of uncertainties of the transition point), fixed patter noise, and non-linearity. Thus, in spite of being the most attractive approach to dynamic range enhancement, variation of the conversion gain has not been practically realized for linear image sensors.